A. Field of the Invention
This invention relates generally to shredding machines, and more particularly to cutting discs for shredding machines which cut sheet materials in transverse directions.
B. Description of the Background Art
Most paper shredders employ a pair of counter-rotating rollers having a plurality of interleaved cutting elements. The cutting elements generally conform to one of two categories, toothed discs and smooth-surface discs of right cylindrica configuration. Shredders employing toothed discs are typically constructed by attaching a plurality of discrete toothed discs and interspersed spacers to a shaft. Shredders employing smooth-surfaced discs are typically constructed by milling a piece of roll stock to form a plurality of spaced apart discs. The latter construction technique is preferable since the entire machining process is conducive for use with fully automated milling machines.
Both types of shredders function similarly. As shreddable material, such as paper, is fed between the counter-rotating rolls, the interleaved cutting elements cut or tear the material into longitudinal strips using a scissor-like action. One example of a smooth-surfaced disc shredder is disclosed in U.S. Pat. No. 3,630,460. The shredder disclosed in this patent includes a plurality of interleaved, counter-rotating discs which cut sheet material into strips using a scissor-like action. The teeth of the toothed discs or grooves in the smooth discs grip the material and pull it between the juxtaposed rollers to produce tension in the material which facilitates shredding. U.S. Pat. No. 3,033,064 discloses a shredder having a plurality of notched discs. The notches grip sheets of paper to advance them between the rollers where the interleaved counter-rotating discs cut the paper into strips.
In many applications, however, such as governmental document destruction, this type of destruction proves inadequate. There is the possibility that the content of these waste documents can be reconstructed since characters remain on the stips. Therefore, each type of shredder has been improved to shred materials in both the longitudinal and lateral directions. U.S. Pat. No. 4,565,330 discloses a toothed disc shredder which uses teeth to draw the sheet materials between the shredding rolls and cut the material in two directions. As the circumferential edges of the discs cut the material into strips, the teeth, in cooperation with a back plate, cut the strips into chips. U.S. Pat. No. 3,860,180 discloses a smooth-surfaced disc shredder including notches formed in the outer periphery of each disc such that the notches are disposed in a helical fashion around each roll. As the circumferential edges of the discs cut the sheet material into strips, the leading edge of the notches cut the material strips into segments.
Although the above-mentioned techniques usually destroy documents satisfactorily, they demonstrate some inadequacies. For example, some shredders use "metal-to-metal" contact to cut strips into segments. This contact causes a significant amount of wear on the discs and rollers. Moreover, this segmenting technique produces relatively more stress between the rollers. Other shredders must hold the sheet material tautly in order for a sharp nose of the trailing edge of the notch to penetrate and cut the material into segments. If the material is loose or too thick, the nose of the notch will not be able to segment the strips. For example, a shredder using notched discs is disclosed in U.S. Pat. No. 4,944,462, which is incorporated by reference herein. This shredder uses notches that are wider at the bottom than at the peripheral edge of the disc. The cutting or shredding action of these discs is significantly superior to those known previously; however, discs of this type experience retention of particles in the notches. These retained particles are randomly discharged from the notches into parts of the shredder increasing the possibility of jamming or fouling other components of the shredder.